Pulsed cathodic protection system and method

ABSTRACT

A cathodic protection system for protecting buried conducting structures, subject to corrosion such as well casings, pipe lines and the like, utilizes a plurality of pulsed D.C. current sources with the negative output terminal of each source connected to a separate structure and the positive output terminal of the sources connected to a common anode located near the structures. A control circuit synchronizes the operation of the several D.C. sources and sets the frequency and width of the output pulses. The amplitude of the output pulses from each D.C. source may be separately adjusted.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a system and method for the cathodicprotection of structures such as pipelines and well casings disposed inan electrically conducting medium such as the ground and moreparticularly to such a system utilizing pulsed D.C. current to protect aplurality of such structures in which the spacing between the structuresand/or different electrical properties of the conducting mediumsurrounding the structures are not amenable to the use of a singlepulsed source.

2. Description of the Prior Art

The use of cathodic protection to prevent corrosion is well establishedfor the protection of metal structures, such as well casings and pipelines, that are buried in conductive soils. Cathodic protection is alsoused for the protection of inner surfaces of tanks which containcorrosive solutions, as well as for the protection of sub-platforms, andother off-shore metal structures. It is well established that thecathodic protection can be accomplished either by the use of sacrificialanodes electrically grounded to the structure to be protected, or by theapplication of low voltage direct current from a power source. In thelatter method steady direct current, half or full wave rectifiedcurrent, and pulsed direct current have all been used.

It has been well established that, when a cathodic protection current isapplied to a circuit including the structure (cathode) to be protectedand its associated anode, a layer of charge is formed at approximately100 A. from the surface of the structure. This layer of charge is calleda taffel double layer. This layer acts as a capacitor in series with theanode-cathode circuit. In the absence of a cathodic protection systemthe soil or other conductive corrosive medium to which a ferrous metalstructure such as a steel pipeline is exposed will cause an adversechemical reaction in which ferrous or iron molecules pass into solutionas positive ions by surrendering electrons to the structure. Hydrogenions in the solution will accept the free electrons and form a gas, e.g.H₂, adjacent to the surface of the structure. Oxygen molecules andcertain other substances, if present in the solution, will also acceptthe electrons. This action results in a loss of iron in the structurewith a consequent degradation of structural integrity.

Direct current cathodic protection systems prevent (or inhibit) the ironmolecules from passing into solution by providing an exterior source offree electrons to the structure. The electrons supplied by the cathodicprotection systems reduce any oxygen molecules and/or hydrogen ionspresent at the surface of the structure. The iron molecules areinhibited from going into solution, because the hydrogen ion and oxygenmolecule receptors for the iron molecule electrons have been reduced bythe cathodic protection system electrons. As a general rule, the greaterthe amount of current (accumulated electrons per unit of time) that issupplied by the cathodic protection system, the greater will be the areaof structure protected.

A typical steady state 15 volt and 15 ampere D.C. cathodic protectionsystem offers good protection but provides only a limited umbrella ofprotection or throw along the structure such as a pipeline to beprotected. Such steady state systems thus require a considerable numberof protection stations for a given length of the structure or pipe to beprotected. Increasing the amount of current supplied by increasing thevoltage, will increase the throw. The average current must, however, belimited such that an excess of hydrogen gas is not generated at thepoint of application of the cathodic protection system. An excess ofhydrogen may cause damage to protective coatings. Excess hydrogen willalso permeate the pipe wall, causing certain pipe materials to crack orrupture.

It has been shown that a pulsed D.C. voltage source having an output ofthe order of 100-300 volts for 5-100 microseconds (“μs”) with a dutycycle of the order of 10% provides a much greater coverage (or throw)per station e.g. one station every few miles of pipeline. Such pulsedsystems have been considered to be particularly effective because,although the average current is still in the order of magnitude of 15amperes, the peak current, which is flowing for a sufficient length oftime to cause the protective reactions to take place, will be typicallyas high as 300 amperes. The pulsed D.C. systems also cause a greaterredistribution of the current along the structure, such as a pipeline,because of the inductive and capacitive reactance of the anode andstructure system.

Copper-copper sulfate electrodes are conventionally used to determinethe effectiveness of cathodic protection systems in protecting wellcasings and pipelines. Such electrodes, comprising a copper rod immersedin a copper sulfate solution (typically a gel) are placed in the ground,adjacent the well casings or pipeline (e.g., 1 or 2 feet there from) andthe potential between the metal structure and the copper rod ismeasured. A potential, typically called “the well head potential”, ofabout 1 volt is considered to provide appropriate protection.

Prior art cathodic protection systems are disclosed in my prior U.S.Pat. Nos. 3,612,898; 3,692,650; and 5,324,405 (“'405 patent”). The '405patent teaches an improvement over the systems disclosed in the earlierpatents in terms of increasing the current distribution or throw of thecurrent along a pipeline or well casing as well as increasing theprotection of neighboring pipelines or well casings. This improvement isaccomplished by the limiting current flow in the power supply throughthe use of back emf current limiting means. The disclosure of the '405patent is incorporated herein by reference.

A typical prior art pulsed protection system is illustrated in FIG. 1 ofthe drawings where reference numerals 10, 12 and 14 designate a D.C.voltage source, an anode/cathode voltage switch and a pulsewidth/frequency control unit, respectively. The positive output issupplied to an anode unit 16 (which may comprise several discrete metalcylinders connected in parallel) via a positive terminal 18 and thenegative output is supplied to a plurality of well casings or pipelines20 and 22 via the negative terminal 24. A diode 25 (or a back emflimiter as taught in the '405 patent) is connected across the outputterminals 18 and 24. The voltage and current waveforms V and I of theoutput, appearing across the terminals 18 and 24, are shown in FIG. 1 tothe right of the switch 12. As is pointed out in the '405 patent the useof diode 25 protects the voltage source from reverse voltage spikes atthe expense of somewhat limiting the current throw and the protectionfor neighboring structures where a single current source is used.

A problem has arisen when a single pulsed D.C. source is used to protecttwo or more structures from a single anode unit where the spacialdistances between the structures and/or the electrical properties of thesoil or other conducting medium result in one or more structuresreceiving excessive current while others receive inadequate current forprotective purposes. The use of a separate anode unit and pulsed sourcesfor each neighboring well casing or pipeline has its own set of problemsas is alluded to in the '405 patent. An under protected well casing orpipeline located in adverse soil conditions may need frequentreplacement. The cost of replacing a damaged well casing or section ofpipeline can be very expensive. For example, the cost to replace a deepwell casing may run as much or more than one million dollars. Thus, theproblem has serious economic consequences.

There is a need for an improved cathodic protection system capable ofadequately protecting multiple adjacent structures such as well casingsand the like which are not amenable to the use of a single pulsedsource.

SUMMARY OF THE INVENTION

A system for the effective cathodic protection of a plurality of spacedelectrically conducting structures such as ferrous metal pipe lines orwell casings exposed to an electrically conducting medium, such as theground, in accordance with the present invention comprises a pluralityof pulsed D.C. current sources with each source being adapted to beconnected to a separate structure. Each current source is arranged tosupply a current pulse of a controllable amplitude to the associatedstructure at a selected frequency. A control circuit is coupled to eachcurrent source and arranged to synchronize the operation of the currentsources so that the current pulses of all current sources occupysubstantially the same time frame during each cycle. In other words,each of the current pulses during a cycle is initiated at substantiallythe same time and the decay of each of the current pulses begins at thesame time. The magnitude of the current from each of the current sourcesmay be separately adjusted to provide the proper amount of current toeach structure to ensure its protection. By the same token, the pulsewidth and cycle frequency of all the current sources may be adjusted asdesired.

It is to be noted that it is the rise or rise time of the current pulsesfrom the several pulsed D.C. current sources which is controlled tooccur during the same time frame. The decay of the current pulses isdependant on the impedance of the load, i.e., the anode, cathode (orwell casing, pipelines etc.) and the intervening conducting medium suchas the soil. The term current rise or current rise time refers to thetime frame in which the current pulse is initiated until the currentpulse begins to decay. Thus, the terminology setting the pulse width ofthe current pulses means setting the current use time for such pulses.

The construction and operation of the present invention can best beunderstood by the following description taken in conjunction with theaccompanying drawings in which like components are designated by likereference numerals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a state of the art pulsed cathodicprotection system in which current pulses from a single source areapplied between a single anode unit and two buried structures, such aswell casings;

FIG. 2 is a block diagram of a cathodic protection system for protectinga plurality of structures, such as well casings or pipe lines, with theuse of multiple pulsed D.C. sources, in accordance with the presentinvention;

FIG. 3 is a block diagram of several components of a pulsed currentsource;

FIG. 4 is a circuit diagram, in block and schematic form, of a pulsedcurrent source utilizing a D.C. to D.C. converter for controlling thecurrent amplitude of the output pulse in accordance with the presentinvention;

FIG. 5 is a schematic/block diagram of a D.C. to D. C. converter; and

FIG. 6 is a circuit diagram partially in block and schematic form ofanother embodiment of a pulsed current source suitable for use in theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to FIG. 2 a cathodic protection system, in accordance withthe present invention, comprises a group of pulsed D.C. sources 26, 28,30 and 32 with each source having a negative output terminal 26b, 28b,30b and 32b arranged to be connected to a separate ferrous metalstructure such as a well casing (or pipe line) 34, 36, 38 and 40 asillustrated.

The positive output terminals 26a, 28a, 30a, and 32a of the D.C. sourcesare connected to an anode unit, as shown, which is submersed in the sameelectrically conducting medium as the well casings, e.g., the ground. Afrequency and pulse width control circuit 42 is connected to each of thepulsed D.C. sources to set the width of the voltage and current pulsesas well as the frequency of such pulses produced across the outputterminals.

The control circuit 42 may include manually controllable knobs 42b and42c for setting the frequency and pulse width of the voltage and currentoutput pulses from the pulsed sources. The waveform of the voltageacross the output terminals of the D.C. source 26 is shown at 26e in thediagram in the left hand portion of FIG. 2 with the generally squarewave output voltage pulses occurring during the same time frame duringeach cycle i.e., t_(o) to t₁, t₂ to t₃ etc. The output voltage pulsesfrom the other pulsed D.C. sources, although not shown, will also be inthe form of square waves and occupy the same time frame during eachcycle as the pulses from the source 26. The current pulses (i.e., risetimes) supplied by the D.C. sources to the several well casings 34, 36,38 and 40 and anode unit 41, which occupy the same time frame as thevoltage pulses, are designated as i₁, i₂, i₃ and i₄, as illustrated. Aspointed out previously, the time frame (or width) of the current pulsesrefers to the rise times of such pulses, i.e., the time from t_(o) tot₁, t₂ to t₃ in the waveform diagram of FIG. 2.

As the impedance between the anode and the well casings increases, dueto increased distance and/or more resistive soil conditions, greatercurrent is required to provide the necessary protection. As isillustrated in the waveform diagram, by way of example, the magnitude ofthe current pulse supplied by the D.C. source 32 is greater than themagnitude of the output current pulse from the D.C. source 26. Theamplitude or magnitude of the output current pulses from each D.C.source is adjustable. The D.C. sources may include manual control meanssuch as knobs 26d, 28d, 30d, and 32d for adjusting the magnitude of theoutput current pulses. There are a myriad of well known and conventionalways to adjust the frequency, pulse width and magnitude of the outputcurrent pulses from the pulsed D.C. sources. If desired, such parameterscould be controlled by a computer.

Once the system of FIG. 2 is installed in the field, well head potentialmeasuring electrodes are typically positioned adjacent the well heads orpipelines which are connected to the pulsed D.C. sources. The desiredpulse width and frequency of the output voltage and current pulses areset by the control circuit 42. The magnitudes of the output currentpulses (typically the mean or average value of the output current) fromthe several D.C. sources are then adjusted until the proper protectionof each well casing is achieved. It should be noted that an adjustmentof the amplitude of the output current from one D.C. source may andprobably will change the current flow from one or more of the other D.C.sources to their associated casings. Thus, it is often necessary to makeseveral successive adjustments of the output current amplitude of theseveral D.C. sources. It should also be noted that it may be necessaryto reset the pulse width and frequency during the adjustment period.

Referring now to FIG. 3 the basic components of a pulsed D.C. sourcesuitable for use in the system are illustrated. A D.C. voltage source44, which may be in the form of a rectified (and filtered) A.C. voltage,is connected to the input of a current amplitude control circuit 46. Theoutput of the amplitude control circuit is supplied to the associatedwell casing or pipeline and the anode unit via an anode/cathode voltageswitch 48. The pulse width and frequency control circuit 42supplies acommon output signal on four output terminals collectively identified as42a to input circuits such as input circuit 26e to control controls theoperation of the associated anode/cathode switch to set the frequencyand width of the output pulses from all of the current sources. Theamplitude of the output current, once set by an operator, is maintainedsubstantially constant by means of a current sensing resistor unit 50connected in a conventional feedback loop well known to those skilled inthe art. It should be noted that the current sensing resistor 50 willtypically include appropriate filtering to provide an output voltagethereacross which is representative of the mean or average current.

A diode 52 is connected across the output terminals for protecting theswitch 48 from high inverse voltages. As is pointed out in the '405patent, this diode may be replaced with a back emf limiter to increasethe current throw at the expense of reverse voltage spikes, if desired.

An additional breakdown of the components for use in a pulsed D.C.source are shown in FIG. 4 wherein an A.C. source supplies current toD.C. to D.C. converter 58 via full wave bridge rectifier 56. The outputof the D.C. to D.C. converter is applied to a group of siliconcontrolled rectifiers (“SCRs”) 60, 62, 64 and 66 which are controlledfrom a frequency control circuit 67 via a conventional trigger circuit68 to form, in conjunction with capacitor 70, a capacitycharge/discharge circuit. The capacitor 70 is connected between theanode/cathode junctions of the SCRs as shown also functions to doublethe voltage from the converter 56. SCRs 60, 66 and 62, 64 are triggeredto conduct alternately in a conventional manner, as is more fullyexplained in the '405 patent. The size (or value) of the capacitor 68sets the pulse width of the output pulses supplied to the load. In thisembodiment the control circuit 62 need only set the frequency andsynchronize the outputs of the several D.C. pulse sources.

The D.C. to D.C. converter is provided with a feedback voltage from acurrent sensing resistor unit 50 to maintain the current output at anadjusted setting.

One type of D.C. to D.C. converter which may be employed is illustratedin FIG. 5 in which the rectified A.C. is filtered via capacitor 74 andapplied to the primary winding of an isolation transformer 76 in serieswith the collector-emitter circuit of a switching power transistor suchas an IGBT. The secondary winding of the transformer supplies the pulsedoutput current through an isolation diode 80 to an anode/cathode voltageswitch and to the negative output terminal. A filter capacitor 82 isconnected across the output terminals as shown.

The current sensing resistor unit 50, connected in series with thenegative output terminal (or positive, if desired) supplies a feedbackvoltage via leads 84, 86 to an amplitude reference circuit 88. Theamplitude of the reference signal in circuit 88 may be adjusted by knob90A (like knob 26 a of circuit 26) connected, for example, to apotentiometer in a conventional manner. The output signal on lead 88 afrom the amplitude reference circuit is representative of the differencebetween the amplitude of the reference signal and the voltage on leads84, 86 which in turn is representative of the mean or average amplitudeof the pulsed current output to the anode unit/well casing. The feedbacksignal on lead 92 is supplied to a pulse width modulator 94 via anisolator circuit 90. The pulse width modulator, which operates at a highfrequency such as 20 to 200 Khz or more to provide accurate control ofthe amplitude of the output current, controls the base or gate electrodeof the switching transistor 78. It should be noted that when used in thepresent application it is not necessary to include the isolationtransformer 76 or diode 80.

It should be noted that if a D.C. to D.C. converter is used with anon-capacitance discharge anode/cathode switch such as a transistor,e.g., an Isolated Gate Bi polar transistor (IGBT), then the controlcircuit must set the pulse width as well as the frequency.

Another example of a pulsed D.C. source is illustrated in FIG. 6 whereinan adjustable current amplitude current control circuit 96 is placed onthe A.C. side of a pulsed D.C. source with a power switching transistor98 such as an IGBT serving as the anode/cathode voltage switch. Atrigger circuit 100 under the control of the frequency and pulse widthcontrol circuit 42 sets the frequency and pulse width of the outputpulses. The current amplitude control circuit 96, which may utilize SCRsor Triacs in a well known manner to adjustably control the portion ofeach half cycle of the input sine wave supplied to the bridge rectifier,receives a feedback signal on lead 101. The feedback signal from thecurrent sensing resistor unit 50 is representative of the load(anode/cathode) current. The control circuit 96, in response to thefeedback signal maintains the value of the adjusted current output tothe bridge rectifier substantially constant.

It should be noted that while an SCR or Triac type amplitude controlcircuit 96 will operate satisfactorily to control the magnitude of thecurrent pulses to the load these circuits are inherently inefficientbecause of power losses in the SCRs or Triacs. In contrast, D.C. to D.C.converters are typically much more efficient due to the low resistancedrop through the switching transistor.

There has thus been described a cathodic protection system and methodfor providing improved protection for multiple structures such as wellcasings or pipelines. While the invention has been described inconnection with several embodiments, it is not intended that the scopeof the invention be limited to such embodiments and examples discussedabove. Various alternatives, modifications, and equivalents will becomeapparent to those skilled in the art without departing from the spiritand scope of the invention as defined by the appended claims.

What is claimed is:
 1. In a system for effecting cathodic protection ofa plurality of spaced electrically conducting structures, includingmetal pipe lines or well casings, exposed to an electrically conductingmedium, including the ground, the medium being in contact with an anodestructure and through which current may be passed to said medium and tothe structures, the combination comprising: a plurality of pulsed D.C.current sources, each source being adapted to be connected to a separateconducting structure for supplying a controllable current at a selectedfrequency pulse between the associated conducting structure and theanode; and a control circuit coupled to each of the current sources forsynchronizing the operation of the current sources so that the currentpulses from the plurality of current sources occupy substantially thesame time frame during each cycle.
 2. The cathodic protection system ofclaim 1 wherein each current source includes means for adjusting themagnitude of the current supplied therefrom.
 3. The cathodic protectionsystem of claim 2 wherein the control circuit adjusts the frequency ofthe current pulses supplied by the D.C. current sources and wherein eachcurrent source includes an anode/cathode switch.
 4. The cathodicprotection system of claim 3 wherein the control circuit sets the pulsewidth and frequency of the current pulses.
 5. The cathodic protectionsystem of claim 4 wherein the anode/cathode switch employs an isolatedgate bipolar transistor to switch the D.C. source across the associatedstructure/anode load.
 6. The cathodic protection system of claim 3wherein the anode/cathode switch includes a plurality of siliconcontrolled rectifirers.
 7. The cathodic protection system of claim 2wherein each current source includes means for maintaining the currentsubstantially constant once the magnitude thereof is set.
 8. Thecathodic protection system of claim 7 wherein the means for maintainingthe current constant includes means for measuring the current output. 9.The cathodic protection system of claim 7 wherein each of the D.C.current sources includes a D.C. to D. C. converter.
 10. The cathodicprotection system of claim 7 wherein each of the D.C. current sourcesinclude a D.C. source for providing an adjustable current level outputand an anode/cathode switch coupled to the control circuit forconnecting the output of the D.C. current source to the associatedstructure and anode unit in accordance with the pulse width andfrequency set by the control circuit.
 11. A method of protecting aplurality of spaced electrically conducting structures, including wellcasings or pipelines, exposed to an electrically conducting mediumincluding the ground, the medium being in contact with the structurescomprising the steps of: immersing an anode unit into the conductingmedium; connecting the negative output terminal of a separate pulsedD.C. source to each conducting structure with each source being adaptedto provide pulsed D.C. output current at a selected frequency and pulsewidth; connecting the positive terminal of each of the pulsed D.C.sources to the anode; and synchronizing the operation of the pulsed D.C.sources so that the current pulses from all of the D.C. sources occupysubstantially the same time frame during each cycle.
 12. The method ofclaim 11 further including the step of individually adjusting themagnitude of the current delivered by the plurality of pulsed D.C.current sources.
 13. The method of claim 12 further including the stepof measuring the well head potential of the plurality of structuresduring the current adjusting step.
 14. The method of claim 11 furtherincluding the step of adjusting the frequency and/or the pulse width ofthe output current pulses.
 15. A cathodic protection system in which oneor more pulsed dc current source protects one or more spacedelectrically conducting structures including metal pipe lines or wellcasings embedded in the ground along with an anode structure comprising:at least one pulsed dc current source, each dc current source having aninput circuit and a pair of output terminals with one terminal adaptedto be connected to an electrically conducting structure and the otherterminal adopted to be connected to the anode structure, dc currentsource being arranged to produce across the output terminals periodiccurrent pulses at a selected frequency and time frame within each cycleas determined by a control signal applied to its input circuit; and afrequency control circuit having a plurality of output terminals andbeing arranged to produce a common controllable frequency output signalon each of the output terminals, one of the output terminals beingconnected to the input circuit of said at least one dc current source,the remaining output terminals being adapted to be connected to theinput circuits of additional pulsed dc current sources.
 16. The cathodicprotection system of claim 15 wherein the frequency control circuit isarranged to produce substantially square wave output signals and whereinsaid at least one dc current source is arranged to produce currentpulses which coincide with the control circuit output signals.
 17. Thecathodic protection system of claim 16 wherein said at least one dccurrent source includes an anode/cathode switch and wherein theanode/cathode switch employs an isolated gate bipolar transistor.
 18. Ina system adapted to affect cathodic protection of a plurality of spacedelectrically conducting structures including metal pipe lines or wellcasings, exposed to an electrically conducting medium including theground, the medium being in contact with an anode structure and throughwhich current may be passed to said medium and to the structures thecombination comprising: one pulsed dc current source having a pair ofoutput terminals and an input circuit, one of the output terminals beingadapted to be connected to one of the conducting structures of the outerterminal being adapted to be connected to the anode, the dc source beingresponsive to the application of a control signal to the input circuitthereof for supplying current pulses at a selected frequency and timeframe within each cycle between the associated conducting structure andthe anode, and a control circuit having a plurality of output terminals,one of the output terminals being connected to the input terminal ofsaid dc source, the other output terminal being adapted to be connectedto the input terminals of other dc sources.
 19. In a system foraffecting cathodic protection of a plurality of spaced electricallyconducting structures, including metal pipe lines or well casings,exposed to an electrically conducting medium, including the ground, themedium being in contact with an anode structure and through whichcurrent may be passed to said medium and to the structures by aplurality of pulsed dc current sources with each source having an inputterminal and a pair of output terminals adapted to be connected to aseparate conducting structure and the anode, each dc current sourcesupplying controlled current pulses at a selected frequency between theassociated conducting structure and the anode in response to a signal onthe input terminal thereof, the combination comprising: one of said dcsources and a control circuit, the control circuit having a plurality ofoutput terminals, one of the output terminals connected to the inputterminal of said one dc source, the other output terminals being adaptedto be connected to an input terminal of a separate dc source, thecontrol circuit providing a common output signal on the output terminalsfor synchronizing the operation of a plurality of dc current sources sothat the current pulses from the plurality of current sources occupysubstantially the same time frame during each cycle.